1. Field of the Invention
The present invention relates to a method of bandpass sampling using single sideband conversion, and more particularly, to a method of bandpass sampling using single sideband conversion which is capable of improving an efficiency of bandpass sampling by performing a bandpass sampling after converting a double-sideband RF signal to a single-sideband signal in the bandpass sampling process which down-converts the RF signal to a baseband in the frequency domain, which is obtained by removing either upper-sideband or lower-sideband spectrum from a double-sideband spectrum of an RF signal.
2. Description of the Related Art
With current mixture of various wireless communication schemes, researches for a terminal structure which can accommodate multi standards have been in progress. Among other things, a software defined radio (SDR) technique which can reconfigure the entire function of a wireless communication system by reconfiguration of software based on a highly advanced digital signal processing technique. Such SDR can allow to accommodate multiple wireless communication standards in an integrated fashion as a single transmission/receipt system platform by only changing appropriate software modules. Such an SDR technique is standing out as a core terminal technique covering the forthcoming wireless techniques.
An RF hardware platform like in the SDR technique that is designed to handle multiple RF standards together requires a processing capability over a wideband frequency band. However, development and module implementation of the related RF devices having such a wideband frequency characteristic needs to be developed further to satisfy the actual practicability in technical and economical terms. Accordingly, the methods of converting and processing wideband RF signals using advanced signal processing techniques have emerged recently, one of which is the software-based bandpass sampling scheme which down-converts an RF signal into a baseband signal.
A bandpass sampling scheme is a method of shifting a frequency of a modulated bandpass signal by sub-sampling the modulated signal, thereby allowing a lower sampling frequency to be utilized.
FIG. 1 shows an example process of frequency down-conversion of one RF signal in a conventional bandpass sampling technique.
As shown in the figure, a signal input through a wideband antenna 10 is amplified by a wideband low noise amplifier (LNA) 11 and then a signal having a desired RF band is extracted from the amplified signal by bandpass filter 12. An analog-digital converter (ADC) 13 converts an analog signal into a digital signal which is then processed by a subsequent digital signal processor (DSP) 14 for restoration of the originally-transmitted signal.
In addition to the conversion of the analog signal into the digital signal, the ADC 13 performs a frequency down-conversion for converting the RF band signal into a signal at a low frequency band close to DC through the bandpass sampling. This allows a RF bandpass signal to be converted into a baseband signal using a sampling frequency lower than one which can be obtained according to a Nyquist sampling theorem. Accordingly, conventional complicated RF signal processing parts which were used for frequency down-conversion of RF signals can be omitted and a sampling frequency required for ADC can be lowered, thereby a processing burden of a digital processor and a cost can be reduced.
An RF signal consists of an upper sideband spectrum and a lower sideband spectrum, both of which in fact contain the same information. Since the sampling frequency of the bandpass sampling used in frequency down-converting of an RF signal is lower than the Nyquist sampling frequency, a very careful exercise in the sampling should be performed so that the wanted RF signal spectrum is shifted to the predetermined low-frequency baseband while completely avoiding overlapping with any other spectrum that could also be shifted by down-converting bandpass sampling. In particular, if multiple RF signals are to be frequency down-converted simultaneously through one bandpass sampling process, there is much higher possibility of spectrum overlapping with one another in the down-converted baseband. Accordingly, a more strict bandpass sampling process is required for the frequency down-conversion process.
Since a higher sampling frequency is required for bandpass sampling of multiple RF signals, there is a need of a more efficient bandpass sampling method which is capable of lowering the bandpass sampling frequency leading to a reduced burden to ADC performance as well as a reduced processing load to the subsequent signal processing processes.